Method for controlling the speed of an internal combustion engine supercharged by means of a turbocharger

ABSTRACT

A method controls an internal-combustion engine ( 1 ) supercharged by a turbocharger ( 12 ) and including a turbine ( 13 ) and compressor ( 14 ). The control method comprises steps of determining a current reduced-mass-flow rate (QAHR) of the compressor ( 14 ), determining a safety threshold (Mmax_turbo) of the reduced-mass-flow rate (QAHR) that delimits in a “reduced-mass-flow rate/compression ratio” plane an area substantially close to achievement of sonic conditions, and imposing that the reduced-mass-flow rate (QAHR) has to be lower than a safety threshold (Mmax_turbo) of the reduced-mass-flow rate (QAHR).

REFERENCE TO RELATED APPLICATION

This application claims benefit of the filing date of and priority toItalian Patent Application BO2010A 000580 filed on Sep. 27, 2010.

BACKGROUND OF INVENTION

1. Field of Invention

The invention relates, generally, to a method for controlling aninternal-combustion engine and, more particularly, to such an enginethat is supercharged by a turbocharger.

2. Description of Related Art

Some internal-combustion engines are provided with aturbocharger-supercharging system, which can increase the powergenerated by the engine using the enthalpy of the exhaust gases tocompress the intake air from the engine and, therefore, increase thevolumetric efficiency of the intake.

A turbocharger-supercharging system includes a turbocharger providedwith a turbine, which is arranged along an exhaust conduit to rotate athigh speed under the pressure of exhaust gases expelled from the engine,and compressor, which is put in rotation by the turbine and arrangedalong the air-supply conduit compressing the intake air from the engine.

In particular, the useful area of the operating range is limited—on theleft side of a “reduced-mass-flow rate/compression ratio” plane by the“pumping” line and at the right side of the same plane by the so-called“saturation” line. The “pumping” line, therefore, defines a first“forbidden” zone and consists of the location of points where theinternal aerodynamic balance of the compressor is offset and there is aperiodical, noisy, and violent flow-rate refusal to the mouth, witheffects that can be destructive for the blading.

In a turbocharger-supercharging system, it is necessary to maintain theoperating range of the turbocharger within a useful area dependent onthe engine point both for functional reasons (i.e., to avoidmalfunctions or else low performance), and structural reasons (i.e., toavoid damage to the turbocharger).

For example, Patent Application US-A1-2009293477 describes a controlmethod of an internal-combustion engine supercharged by a turbochargerincluding a turbine and compressor. The control method envisagesdetermining the current mass-flow rate of the compressor, determining alower security threshold of the mass-flow rate, and requiring that thecurrent mass-flow rate of the compressor be greater than the securitythreshold of the mass-flow rate. Patent Application US-A1-2009293477,therefore, describes a method that allows preventing damage to theturbocharger, but does not allow optimizing the performance of theturbocharger itself.

Patent Application EP1741895A1 describes a control method of aninternal-combustion engine supercharged by a turbocharger including acompressor, a turbine adapted to drive into rotation the compressorunder the action of the exhaust gases of the engine, and a waste-gatevalve adapted to regulate the flow of exhaust gases supplied as input tothe turbine for controlling the speed of rotation of the turbine itselfaccording to an output supercharged-target pressure required by thecompressor.

The control method described in Patent Application EP1741895A1 includesthe steps of: measuring the pressure of intake air entering thecompressor; determining the mass-flow rate of the compressor;calculating—through a predetermined map that characterizes the operationof the compressor and, according to the preset-limit speed of rotation,measured air pressure and mass-flow rate—a supercharging-limit pressure,which is related to the obtainable output-air pressure from thecompressor when the turbine rotates at a speed substantially equal tothe preset-limit speed; verifying if a requested supercharged-targetpressure satisfies a preset relation with a supercharged-limit pressurecalculated in the case where the relationship is met, and actuating thewaste-gate valve for controlling the speed of rotation of the turbineaccording to the supercharged-limit pressure, thus reducing therotational speed of the turbocharger at a value substantially equal tothe preset-limit speed.

Patent Application EP1741895A1 indicates that, in supercharging systemsof the above-described type, it is necessary to be able to limit, at thevarying of operating conditions of the engine, the maximum rotationspeed of the turbocharger both for functional and structural reasons soas to avoid critical operating conditions that may cause damage to theturbocharger. However, it gives no indication on how to implement thelimitation of the maximum rotation speed of the turbocharger.

Patent Application EP2014894A1 describes instead a control method of aninternal-combustion engine supercharged by a turbocharger provided witha turbine and compressor that envisages providing in a“Reduced-Mass-Flow Rate/Compression Ratio” plane at least one “operatinglimit” curve, at least one “interaction” curve of a waste-gate valveregulating a bypass conduit of the turbine, and at least one“intervening” curve of a Poff valve regulating a bypass conduit of thecompressor. The control method according to Patent ApplicationEP2014894A1 envisages the use of the “operating limit” curve forlimiting the pressure target downstream of the compressor used by themotor control. The method further envisages controlling the opening ofthe waste-gate valve if the “intervening” curve of the waste-gate valveis exceeded and Poff valve if the “intervening” curve of the Poff valveis exceeded. The control method described by Patent ApplicationEP2014894A1 is able to ensure that the operating range of theturbocharger remains within the useful area in any working condition ofthe internal-combustion engine.

The so-called “saturation” line defines a second “forbidden” zone,corresponds to the reaching of sonic conditions (and consequent blockingof the flow) entering the turbine, and defines the maximum possible flowthat the compressor can provide in the given conditions of the intakeenvironment. Substantially close to the “saturation” line, theturbocharger reaches, therefore, very high speeds and is able to developthe maximum power for compressing air intake from the engine and, thus,increasing the volumetric efficiency of the aspiration. Unfortunately,however, substantially close to the “saturation” line, due to the highspeeds involved, it may occur that the turbocharger accelerates out ofcontrol until reaching the sonic block, with destructive effects uponthe turbocharger itself.

Thus, there is a need in the related art for a control method of aninternal-combustion engine supercharged by a turbocharger. Morespecifically, there is a need in the related art for such a method thatis inexpensive and simple to implement. There is a need in the relatedart for such a method that also ensures that the operating range of theturbocharger remains within the useful area substantially close to the“saturation” line, but without reaching the sonic block.

SUMMARY OF INVENTION

The invention overcomes the disadvantages in the related art in a methodfor controlling an internal-combustion engine supercharged by aturbocharger and including a turbine and compressor. The control methodincludes steps of determining a current reduced-mass-flow rate of thecompressor, determining a safety threshold of the reduced-mass-flow ratethat delimits in a “reduced-mass-flow rate/compression ratio” plane anarea substantially close to achievement of sonic conditions, andimposing that the reduced-mass-flow rate has to be lower than a safetythreshold of the reduced-mass-flow rate.

One advantage of the method for controlling an internal-combustionengine supercharged by a turbocharger of the invention is that it isinexpensive and simple to implement.

Another advantage of the method for controlling an internal-combustionengine supercharged by a turbocharger of the invention is that it doesnot use a high computing power of the electronic-control unit.

Another advantage of the method for controlling an internal-combustionengine supercharged by a turbocharger of the invention is that it doesnot require the installation of additional components (in particular,sensors or actuators) with respect to those already present in a moderninternal-combustion engine.

Another advantage of the method for controlling an internal-combustionengine supercharged by a turbocharger of the invention is that itensures that the operating range of the turbocharger remains within theuseful area substantially close to the “saturation” line, but withoutreaching the sonic block.

Other objects, features, and advantages of the method for controlling aninternal-combustion engine supercharged by a turbocharger of theinvention are readily appreciated as the control method becomes moreunderstood while the subsequent detailed description of at least oneembodiment of the control method is read taken in conjunction with theaccompanying drawing thereof.

BRIEF DESCRIPTION OF EACH FIGURE OF DRAWING

FIG. 1 diagrammatically shows an internal-combustion engine superchargedby a turbocharger and provided with an electronic-control unit thatimplements a method for controlling the engine according to anembodiment of the invention;

FIG. 2 shows “characteristic” curves of a compressor of the turbochargerdiagrammatically shown in FIG. 1 on a “Reduced-Mass-FlowRate/Compression Ratio” plane; and

FIGS. 3-6 show a “Reduced-Mass-Flow Rate/Compression Ratio” plane thatillustrates “limit of operation” and “intervening” curves used in theembodiment of the method for controlling an internal-combustion enginesupercharged by a turbocharger of the invention implemented by theelectronic-control unit diagrammatically shown in FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENT(S) OF INVENTION

In FIG. 1, an internal-combustion engine, generally indicated at 1, issupercharged by a turbocharger-supercharging system 2. Theinternal-combustion engine 1 has four cylinders 3, each of which isconnected to an intake manifold 4 by way of at least one respectiveintake valve (not shown) and exhaust manifold 5 by way of at least onerespective exhaust valve (not shown). The intake manifold 4 receivesfresh air (i.e., air coming from the external environment) through asuction conduit 6, which is provided with an air filter 7 and regulatedby a throttle valve 8. Along the suction conduit 6, an intercooler 9 isprovided a function of which is to cool the intake air. To the exhaustmanifold 5 is connected an exhaust conduit 10 that feeds the exhaustgases produced by the combustion to an exhaust system, which emits thegases produced from burning into the atmosphere and usually includes atleast one catalyst 11 and at least one silencer (not shown) disposeddownstream of the catalyst 11.

The supercharged system 2 of the internal-combustion engine 1 includes aturbocharger 12 provided with a turbine 13 that is arranged along theexhaust conduit 10 to rotate at high speed under the action of exhaustgases expelled from the cylinders 3 and a compressor 14 that is arrangedalong the suction conduit 6 and is mechanically connected to the turbine13 to be driven into rotation by the turbine 13 itself so as to increasethe air pressure fed into the supply conduit 6.

Along the exhaust conduit 10 a bypass conduit 15 is provided, which isconnected in parallel to the turbine 13 so as to present its endsconnected upstream and downstream of the turbine 13 itself. Along thebypass conduit 15 a waste-gate valve 16 is arranged, which is adapted toregulate the flow of exhaust gases flowing through the bypass conduit 15and is driven by an actuator 17. Along the intake conduit 6 a bypassconduit 18 is provided, which is connected in parallel with thecompressor 14 so as to present its ends connected upstream anddownstream of the compressor 14 itself. Along the bypass conduit 18 aPoff valve 19 is arranged, which is adapted to regulate the flow ofexhaust gases flowing through the bypass conduit 18 and is driven by anactuator 20.

The internal-combustion engine 1 is controlled by an electronic-controlunit 21, which directs the operation of all components of theinternal-combustion engine 1 including the supercharging system 2. Inparticular, the electronic-control unit 21 drives the control actuators17, 20 of the waste-gate valve 16 and of the Poff valve 19. Theelectronic-control unit 21 is connected to sensors 22 that measure thetemperature T_(o) and pressure P_(o) along the intake conduit 6 upstreamof the compressor 14, to sensors 23 that measure temperature andpressure along the suction conduit 6 upstream of the throttle valve 8,and to sensors 24 that measure temperature and pressure inside thesuction manifold 4. In addition, the electronic-control unit 21 isconnected to a sensor 25 that measures the angular position (and hencethe rotation speed) of a crankshaft of the internal-combustion engine 1and a sensor 26 that measures the phase of the intake and/or dischargevalves. There are no provided sensors adapted for measuring the rotationspeed of the turbocharger 12.

Among other things, the electronic-control unit 21 maintains theoperating range of the turbocharger 12 within a useful area. Thefollowing shows the control mode used by the electronic-control unit 21to keep the operating range of the turbocharger 12 in a useful area andto prevent the turbocharger 12 from reaching sonic conditions in thevicinity of a “saturation” line 35 (illustrated in FIGS. 4 and 5).

During a phase of design and development of the internal-combustionengine 1, the characteristic curves of the compressor 14 are analyzed(provided by the manufacturer of turbocharger 12) in a“Reduced-Mass-Flow Rate/Compression Ratio” plane. An example of thecharacteristic curves of a commercial compressor 14 is illustrated inFIG. 2.

The characteristic curves shown in FIG. 2 are normalized to an absolutereference temperature T_(o) _(—) _(rif) and an absolute referencepressure P_(o) _(—) _(rif). On the left side of the “Reduced-Mass-FlowRate/Compression Ratio” plane is a first “forbidden” zone delimited bythe “pumping” line, consisting of the location of points wherein theinside aerodynamic balance of the compressor 14 is offset and there is aperiodical, noisy and violent flow refusal to the mouth, with effectsthat can be destructive for the blading.

Instead, in the right side of the “Reduced-Mass-Flow Rate/CompressionRatio” plane there is a second “forbidden” zone delimited by theso-called “saturation” line 35 (shown in FIGS. 4 and 5), whichcorresponds to the attainment of sonic conditions (and, thus, blockingthe flow) at the entrance of the turbine 13 and defines the maximumpossible flow that the compressor 14 can provide in the given conditionsof the suction environment.

According to that shown in FIG. 3, through an analysis of thecharacteristic curves of the compressor 14 a curve 27 is determinedlimiting the rotation speed of the turbocharger 12 and a curve 28 thatdelimits the pumping of the turbocharger 12. According the curves 27, 28two “operating limit” curves 29, 30 are established that are used torestrict the target of pressure downstream of the compressor 14 used bythe engine control. For determining the “operating limit” curve 29 athreshold S1 (constant or variable) is determined establishing thedistance between the “operating limit” curve 29 and the curve 27 thatlimits the rotation speed of the turbocharger 12; similarly, todetermine the “operating limit” curve 30 a threshold S2 (constant orvariable) is determined establishing the distance between the “operatinglimit” curve 30 and the curve 28, which delimits the pumping of theturbocharger 12.

Moreover, according to the curves 27, 28 two “intervening” curves 31, 32are established of the waste-gate valve 16 regulating the bypass conduit15 of the turbine 13 and two “intervening” curves 33, 34 of the Poffvalve 19 that regulates the bypass conduit 18 of the compressor 14. Todetermine the “intervening” curve 31 of the waste-gate valve 16 athreshold S₃ (constant or variable) is determined establishing thedistance between the “operating limit” curve 29 and the “intervening”curve 31 of the waste-gate valve 16; similarly, to determine the“intervening” curve 32 of the waste-gate valve 16 a threshold S₄(constant or variable) is determined establishing the distance betweenthe “intervening” curve 32 of the waste-gate valve 16, and the curve 28,which delimits the pumping of the turbocharger 12. To determine the“intervening” curve 33 of the Poff valve 19 a threshold S₅ (constant orvariable) is determined establishing the distance between the “operatinglimit” curve 29 and the “intervening” curve 33 of the Poff valve 19;similarly, to determine the “intervening” curve 34 of the Poff valve 19a threshold S₆ (constant or variable) is determined establishing thedistance between the “intervening” curve 34 of the Poff valve 19 and thecurve 28, which delimits the pumping of the turbocharger 12.

During operation of the internal-combustion engine 1, theelectronic-control unit 21 uses the “operating limit” curves 29, 30 tolimit the pressure target downstream of the compressor 14 used by theengine control. In other words, the engine control unit implemented inthe electronic-control unit 21 determines in a known way and as afunction of the engine point, a target pressure downstream of thecompressor 14, which represents a desired and optimal pressure valuedownstream of the compressor 14. If the target pressure downstream ofthe compressor 14 is compatible with the “operating limit” curves 29, 30then the target pressure downstream of the compressor 14 is maintained,otherwise if the target pressure downstream of the compressor 14 is notcompatible with “operating limit” curves 29, 30 then the target pressuredownstream of the compressor 14 is limited to a maximum value compatiblewith the “operating limit” curves 29, 30.

In particular, to limit the target pressure downstream of the compressor14 the reduced-mass-flow rate Q_(AH) of the compressor 14 is determined,according to the current reduced-mass-flow rate Q_(AH) of the compressor14 the maximum possible compression ratio RC is determined by using the“operating limit” curves 29, 30, the maximum possible pressure isdetermined downstream of the compressor 14 by multiplying the absolutepressure P_(o), upstream of the compressor 14 for the maximum possiblecompression ratio RC, and the target pressure downstream of compressor14 is limited to the maximum possible pressure downstream of thecompressor 14 if the target pressure downstream of the compressor 14 isgreater than the possible maximum pressure downstream of the compressor14.

The reduced-mass-flow rate Q_(AHR) of the compressor 14 is determinedusing the following equation:

$Q_{AHR} = {Q_{AH} \cdot \sqrt{\frac{T_{o}}{T_{orif}}} \cdot \frac{P_{orif}}{P_{o}}}$whereinQ_(AH)=mass-flow rate of the compressor 14;Q_(AHR)=reduced-mass-flow rate of the compressor 14;T_(o)=absolute temperature upstream of the compressor 14;P_(o)=absolute pressure upstream of the compressor 14;T_(o) _(—) _(rif)=absolute reference temperature; andP_(o) _(—) _(rif)=absolute reference pressure.

The absolute reference temperature T_(o) _(—) _(rif) and the absolutereference pressure P_(o) _(—) _(rif) are the conditions in which werederived characteristic curves of the compressor 14 and, therefore, ofthe curves 27-34 and are project data known in advance. The absolutetemperature T_(o) upstream of the compressor 14 and the absolutepressure P_(o) upstream of the compressor 14 are measured by sensors 22.The mass-flow rate Q_(AH) of the compressor 14 can be measured by aspecifically dedicated flow rate sensor or can be estimated in a knownway by the electronic-control unit 21.

According to a different embodiment not illustrated, the measure of theabsolute temperature T_(o) upstream of the compressor 14 (i.e.,substantially room temperature) may not be provided; in this case thereduced-mass-flow rate Q_(AHR) can be “partially” normalized on thebasis of the relation between the pressure P_(o)/P_(o) _(—) _(rif)without taking into account the relation between the temperatures T_(o)and T_(o) _(—) _(rif).

The curves 28, 30, 32, 34 are independent from the reduced limit speedN_(tcR) of the turbocharger 12, while the curves 27, 29, 31, 33 aredependent on the reduced limit speed N_(tcR) of the turbocharger 12(i.e., vary at the varying of the reduced limit speed N_(tcR) of theturbocharger 12). In other words, for the turbocharger 12 is set a limitspeed N_(tc) preset by the turbocharger 12 above which the turbocharger12 is brought in a critical condition; using the preset-limit speedN_(tcR) of the turbocharger 12 the current reduced limit speed N_(tcR)of the turbocharger 12 is calculated on the basis of the absolutetemperature T_(o) upstream of the compressor 14 using the followingequation:

$N_{tcR} = {N_{tc} \cdot \sqrt{\frac{T_{orif}}{T_{o}}}}$whereinN_(tc)=limit speed of turbocharger 12;N_(tcR)=reduced limit speed of turbocharger 12;T_(o)=absolute temperature upstream of the compressor 14; andT_(o) _(—) _(rif)=absolute reference temperature.

At the varying of the absolute temperature T_(o) upstream of thecompressor 14 and at the same preset-limit speed N_(tc) of theturbocharger 12 the current reduced limit speed N_(tcR) of theturbocharger 12 varies; therefore, the electronic-control unit 21cyclically determines according to the absolute temperature T_(o)upstream of the compressor 14 and according to the preset-limit speedN_(tc) of the turbocharger 12 (which always remains constant) thecurrent reduced limit speed N_(tcR) of the turbocharger 12 and accordingto the current reduced limit speed N_(tcR) of the turbocharger 12 isable to determine the curves 27, 29, 31, 33 to be used. Alternatively,since the preset-limit speed N_(tc) of the turbocharger 12 is constantto simplify the management of the curves 27, 29, 31. 33, the curves 27,29, 31, 33 themselves may be stored in the electronic-control unit 21and parameterized according to the absolute temperature T_(o) upstreamof the compressor 14; in this way, the electronic-control unit 21 doesnot need to calculate the current reduced limit speed N_(tcR) of theturbocharger 12 and, therefore, choose the curves 27, 29, 31, 33 to beused, but simply needs to update the curves 27, 29, 31, 33 as a functionof the absolute temperature T_(o) upstream of the compressor 14.

According to another simplified embodiment (and, therefore, lessaccurate), instead of using the current reduced-mass-flow rate Q_(AHR)the current mass-flow rate Q_(AH) (not reduced) or the target mass-flowrate Q_(AHR) (reduced or not reduced) could be used.

Once determined, the current reduced limit speed N_(tcR), theelectronic-control unit 21 is prepared to determine a critical thresholdM_(critica) of the reduced-mass-flow rate Q_(AHR). As best illustratedin FIG. 4, said critical threshold M_(critica) delimits in the“reduced-mass-flow rate/compression ratio” plane a portion of the usefularea of the operating range of the turbocharger 12 that is hereinafterreferred to as the “critical area,” as while remaining within the usefularea it represents the area substantially close to the attainment ofsonic conditions (i.e., substantially close to the “saturation” line35). The critical zone is characterized by the collapse of theefficiency of the compressor 14 and by a high instability of the speedof the turbocharger 12, which may dangerously accelerate.

The critical threshold M_(critica) is variable depending on the reducedlimit speed N_(tcR) (as best illustrated in FIG. 5).

To reduce the instability characterizing the critical zone, theelectronic-control unit 21 is arranged to filter the currentreduced-mass-flow rate Q_(AHR) used to limit the pressure targetdownstream of the compressor 14. Similarly, the electronic-control unit21 is arranged to filter the pressure target downstream of thecompressor 14. The filtering capacity of the current reduced-mass-flowrate Q_(AHR) used to limit the pressure target downstream of thecompressor 14 and the target pressure downstream of the compressor 14 iscapable of reducing the dynamics of the abovementioned variables.According to an embodiment, the filtering is achieved through a low passfirst order filter.

In the case of reduced-mass-flow rate Q_(AHR) exceeding the criticalthreshold M_(critica), the control unit 21 is then prepared forfiltering with a low pass first order type filter both the currentreduced-mass-flow rate Q_(AHR), and the target pressure downstream ofthe compressor 14.

According to an embodiment, the control unit is configured to determinea security threshold M_(max) _(—) _(turbo) of the reduced-mass-flow rateQ_(AHR). As best illustrated in FIG. 4, the security threshold M_(max)_(—) _(turbo) delimits a portion to avoid of the critical area, as it isthe substantially closest to achieve sonic conditions (i.e.,substantially closer to the “saturation” line 35) and represents areduced-mass-flow rate Q_(AHR) beyond which the turbocharger 12 shouldnot go.

The security threshold M_(max) _(—) _(turbo) is greater than thecritical threshold M_(critica). Furthermore, the security thresholdM_(max) _(—) _(turbo) varies depending on the reduced limit speedN_(tCR) (as best illustrated in FIG. 5).

The electronic-control unit 21 is configured to impose that thereduced-mass-flow rate Q_(AHR) of the compressor 14 is lower than thesecurity threshold M_(max) _(—) _(turbo) of the reduced-mass-flow rateQ_(AHR).

According to a further variant, the electronic-control unit 21 isarranged to determine a security threshold N_(max) _(—) _(turbo) of thespeed of the supercharged internal-combustion engine 1. The securitythreshold N_(max) _(—) _(turbo) of the speed of the superchargedinternal-combustion engine 1 is in turn determined as a function of thesecurity threshold M_(max) _(—) _(turbo) of the reduced-mass-flow rateQ_(AHR).

In particular, the security threshold N_(max) _(—) _(turbo) of the speedof the supercharged internal-combustion engine 1 is calculated using thefollowing equation:

$N_{{ma}\; x\;\_\;{turbo}} = {M_{m\;{ax}\;\_\;{turbo}} \cdot \sqrt{\frac{T_{o\;\_\;{rif}}}{T_{o}}} \cdot \frac{P_{o\;}}{P_{\;{o\;\_\;{rif}}}} \cdot \frac{1}{\left( {m \cdot 30 \cdot N_{cil}} \right)}}$whereinN_(max) _(—) _(turbo)=security threshold of the speed of thesupercharged internal-combustion engine 1;M_(max) _(—) _(turbo)=security threshold of the reduced-mass-flow rateQ_(AHR);T_(o)=absolute temperature upstream of the compressor 14;P_(o)=absolute pressure upstream of the compressor 14;T_(o) _(—) _(rif)=absolute reference temperature;P_(o) _(—) _(rif)=absolute reference pressure;N_(cil)=number of cylinders 3 of the internal-combustion engine 1; andM=mass of air drawn for each cylinder 3 of the internal-combustionengine 1.

The security threshold N_(max) _(—) _(turbo) is used to limit the speedof the supercharged internal-combustion engine 1, so that the currentreduced-mass-flow rate Q_(AHR) is lower than the threshold M_(max) _(—)_(turbo).

According to an embodiment, for the turbocharger 12 a preset speed limitN_(tc) of the turbocharger 12 is set above which the turbocharger 12 isbrought into a critical condition; using the preset speed limit N_(tc)of the turbocharger 12 the current reduced speed limit N_(tCR) of theturbocharger 12 is calculated based on the absolute temperature T_(o)upstream of the compressor 14 using the following equation:

$N_{tcR} = {N_{tc} \cdot \sqrt{\frac{T_{orif}}{T_{o}}}}$whereinN_(tc)=speed limit of the turbocharger 12;N_(tCR)=reduced speed limit of the turbocharger 12;T_(o)=absolute temperature upstream of the compressor 14; andT_(o) _(—) _(rif)=absolute reference temperature.

At the varying of absolute temperature T_(o) upstream of the compressor14 and at the same preset-limit speed N_(tc) of the turbocharger 12 thecurrent reduced limit speed N_(tCR) of the turbocharger 12 varies;therefore, the electronic-control unit 21 cyclically determinesaccording to the absolute temperature T_(o) upstream of the compressor14 and according to the preset-limit speed N_(tc) of the turbocharger 12(which always remains constant) the current reduced limit speed N_(tCR)of the turbocharger 12 and according to the current reduced limit speedN_(tCR) of the turbocharger 12 is able to determine the curves 27, 29,31, 33 to be used. Alternatively, since the preset-limit speed N_(tc) ofthe turbocharger 12 is constant to simplify the management of the curves27, 29, 31, 33, the curves 27, 29, 31, 33 themselves may be stored inthe electronic-control unit 21 and parameterized according to theabsolute temperature T_(o) upstream of the compressor 14; in this way,the electronic-control unit 21 does not need to calculate the currentreduced limit speed N_(tCR) of the turbocharger 12 and, therefore,choose the curves 27, 29, 31, 33 to be used, but simply needs to updatethe curves 27, 29, 31, 33 as a function of absolute temperature T_(o)upstream of the compressor 14.

According to what has been described insofar and as shown in FIG. 5, thecurrent reduced speed limit N_(tCR) varies depending on several factors,in particular, on the absolute temperature T_(o) upstream of thecompressor 14.

According to an embodiment, in a preliminary stage of setting and tuninga lower limit speed of the turbocharger 12 and a higher limit speed ofthe turbocharger 12 are preset (which represents a limit of theturbocharger 12 beyond which is best to not exceed in order to notcontract serious breakage or damage to the turbocharger 12 itself). Inuse, these two values are used to calculate a reduced lower limit speedN_(rid) _(—) _(inf) of the turbocharger 12 (which is calculated usingthe formula described above, and varies according to the preset lowerlimit speed of the turbocharger 12 and the absolute temperature T_(o)upstream of the compressor 14) and a reduced higher limit speed N_(rid)_(—) _(sup) of the turbocharger 12 (which is also calculated using theformula described above, is greater than the reduced lower speed limitof the turbocharger 12 and varies according to the preset-limit speed ofthe turbocharger 12 and of the absolute temperature T_(o) upstream ofthe compressor 14). The reduced lower limit speed N_(rid) _(—) _(inf) ofthe turbocharger 12 and the reduced higher limit speed N_(rid) _(—)_(sup) of the turbocharger 12 delimit an overspeed area in the“reduced-mass-flow rate/compression ratio” plane. During the life-spanof the turbocharger 12 it is often the case that the overspeed areamoves in the “reduced-mass-flow rate/compression ratio” plane (forexample, due to the influence of the absolute temperature T_(o) upstreamof the compressor 14). In use, once calculated the current reduced speedlimit, the electronic-control unit 21 is arranged to control theturbocharger 12 to bring the reduced speed limit to a value lower thanthe reduced lower limit speed N_(rid) _(—) _(inf) every time in which avalue of the current reduced limit speed included within the overspeedinterval is detected.

In particular, it is established at a preliminary stage of setting andtuning a first threshold value S_(OV) _(—) ₁ and the turbocharger 12 iscontrolled to bring the reduced speed limit to a value less than thereduced lower limit speed N_(rid) _(—) _(inf) once a time interval haspassed substantially equal to the first threshold value S_(OV) _(—) ₁from the moment a value of the current reduced speed limit includedwithin the overspeed interval (as shown in FIG. 6) is detected. In otherwords, when the electronic-control unit 21 detects a current reducedlimit speed within the overspeed interval, a timer is initialized toreport the reduced limit speed to a value lower than the reduced lowerlimit speed N_(rid) _(—) _(inf) once a time interval has passedsubstantially equal to the first threshold value S_(OV) _(—) ₁ (forexample, by a soft fitting).

According to an embodiment, at a preliminary stage of setting and tuninga second threshold value S_(OV) _(—) ₂ is established. Theelectronic-control unit 21 is arranged to initialize a timer every timethat the reduced limit speed descends below the reduced lower limitspeed N_(rid) _(—) _(inf) and inhibits the operation of the turbocharger12 within the overspeed region for a time interval of duration valuesubstantially equal to the second threshold S_(OV) _(—) ₂.

The first threshold value S_(OV) _(—) ₁ and the threshold value S_(OV)_(—) ₂ are variable according to the state of ageing and wear on thesupercharger 12.

According to an embodiment, the control unit 21 is capable of storingthe total time spent within the overspeed area and inhibit the operationof the turbocharger 12 in the overspeed area for the remaining usefullife of the turbocharger 12 itself once the total time is substantiallyequal to a safe limit value (determined in a preliminary stage ofsetting and tuning).

Moreover, according to an embodiment, the control unit 21 is adapted toinhibit the operation of the turbocharger 12 in the overspeed area whenthe reduced-mass-flow rate Q_(AHR) is above the critical thresholdM_(critica).

According to an embodiment, the threshold S_(OV) _(—) ₂ can varyaccording to the frequency of the most recent overspeed. In other words,the threshold S_(OV) _(—) ₂ is greater the more frequently a currentreduced limit speed is detected within the overspeed range. Thethreshold S_(OV) _(—) ₂ may for example be calculated as follows:S _(OV) _(—) ₂ =f((Σt _(over) _(—) _(speed) −S _(OV) _(—) ₃)/timer)wherein S_(OV) _(—) ₃ is an operator to decrease (e.g., K*timer with Kthat represents a preset coefficient), while the summation of time spentwithin the overspeed interval and the timer are initialized at each tripof the supercharged internal-combustion engine 1 (i.e., typically foreach start/stop cycle of the supercharged internal-combustion engine 1)and the timer is started at the first overspeed. The function is, forexample, in increase.

According to a further variation, for each trip of the superchargedinternal-combustion engine 1 (i.e., for each start/stop cycle thesupercharged internal-combustion engine 1 itself). According to anotherembodiment, as soon as the control unit 21 verifies the operationcondition within the overspeed area, a counter C of the time spent inoverspeed is initialized. The counter C can be calculated using thefollowing formula:C=k1*Σt _(over) _(—) _(speed) −k2*Σt _(NOT) _(—) _(over) _(—) _(speed)wherein K1 and K2 are predetermined coefficients in a preliminary phase,while the summation of time spent within the overspeed interval and thesummation of time spent outside the area of overspeed are initialized ateach trip of the supercharged internal-combustion engine 1.

According to an embodiment, in a preliminary phase of setting and tuninga fourth threshold value S_(OV) _(—) ₄ is established, which is comparedwith the counter C of the time spent in overspeed. In the case ofcounter C greater than or substantially equal to the fourth thresholdvalue S_(OV) _(—) ₄, the control unit 21 is adapted to inhibit theoperation of the turbocharger 12 within the overspeed area. On thecontrary, in case of counter C lower than the fourth threshold valueS_(OV) _(—) ₄, the control unit 21 is set to allow the operation of theturbocharger 12 within the overspeed area (for example, with theassistance of a hysteresis operator).

According to an embodiment, the control unit 21 utilizes the currentpressure of the turbocharger 12 instead of the current reduced limitspeed to recognize the current operation in the overspeed area.

The control method is inexpensive and simple to implement. Also, thecontrol method does not use a high computing power of theelectronic-control unit 21. Furthermore, the control method does notrequire the installation of additional components (in particular,sensors or actuators) with respect to those already present in a moderninternal-combustion engine. In addition, the control method ensures thatthe operating range of the turbocharger remains within the useful areasubstantially close to the “saturation” line, but without reaching thesonic block.

It should be appreciated by those having ordinary skill in the relatedart that the control method has been described above in an illustrativemanner. It should be so appreciated also that the terminology that hasbeen used above is intended to be in the nature of words of descriptionrather than of limitation. It should be so appreciated also that manymodifications and variations of the control method are possible in lightof the above teachings. It should be so appreciated also that, withinthe scope of the appended claims, the control method may be practicedother than as specifically described above.

What is claimed is:
 1. A method for controlling an internal-combustionengine (1) supercharged by a turbocharger (12) provided with a turbine(13) and compressor (14), said control method comprising steps of:determining a current reduced-mass-flow rate (QAHR) of the compressor(14); determining a safety threshold (Mmax_turbo) of thereduced-mass-flow rate (QAHR) that delimits in a “reduced-mass-flowrate/compression ratio” plane an area substantially close to achievementof sonic conditions; and imposing that the current reduced-mass-flowrate (QAHR) has to be lower than a safety threshold (Mmax_turbo) of thereduced-mass-flow rate (QAHR).
 2. A control method as set forth in claim1, wherein said control method comprises further steps of: establishinga predetermined limit speed (N) of the compressor (14); calculating areduced limit speed (NR) of the compressor (14) by using thepredetermined limit speed (N) and an absolute temperature (To) upstreamof the compressor (14); and determining the safety threshold(Mmax_turbo) of the reduced-mass-flow rate (QAHR) according to thereduced limit speed (NR).
 3. A control method as set forth in claim 1,wherein said control method comprises further steps of: determining asafety threshold (Nmax_turbo) of speed of the internal-combustion engine(1) according to the safety threshold (Mmax_turbo) of thereduced-mass-flow rate (QAHR); and imposing that the speed has to belower than the safety threshold (Nmax_turbo) of the speed.